Conventional display monitors typically present video images in the form of a rapid sequence of video fields, changed at a high frequency to create the illusion of motion. Television cameras and other sources of video generally do not produce full-frame images, but instead such video sources typically produce a field consisting of about half of the lines of each full-frame image, at a rate of, for example, 60 such fields per second (in one interlaced system). Alternate fields contain alternate lines of video data. In other words, one field contains the odd numbered lines and the next field contains the even numbered lines. Accordingly, each field of a video image may be identified as an “odd” field or an “even” field.
In a typical interlaced system, the sequence of video fields thus alternates between the odd fields and the even fields. A conventional display monitor receiving the sequence of fields reproduces each video field in the sequence. Each field is displayed on the display screen, such as a television screen, on only half of the scan lines. For example, first an odd field is displayed, using the odd-numbered scan lines, and then an even field is displayed using the even-numbered scan lines, and so on. The display scans a raster across the screen from the top left to the top right producing a first scan line, and then returns the raster to the left edge of the screen to a position slightly below the original position. The position to which the raster returns, however, is not immediately below the first scan line, but allows sufficient space to accommodate an intervening scan line on the alternate field. The raster then scans across to the right edge of the screen to produce a second scan line, and continues in this manner to the bottom edge of the screen.
The distance between the scan lines is a function of the size of the monitor, but generally allows an intervening scan line (the first scan line of the other field) to be drawn after the completion of the first field. The invisible return of the raster to the left edge of the screen after scanning each scan line is a fly-back or horizontal refresh stage that occurs much more rapidly than the visible left-to-right lines. In this manner, approximately 525 active scan lines may be produced (e.g., in the predominate video format of the United States) to complete a single video frame, half of which is displayed in each field.
Once reaching the bottom edge of the screen, the raster is invisibly returned to the original position at the top left corner during a “vertical blanking interval” stage. The horizontal and vertical blanking interval stages are high speed and invisible. With respect to a conventional television, this interlaced video scanning approach is an appropriate compromise between vertical refresh rate, vertical resolution, and limited bandwidth.
Although widely adopted, these methods for alternating between an odd frame and an even frame used by conventional television systems are well known to have various disadvantages, such as line flicker, line crawl, dot crawl, limited horizontal resolution, visible line structure and large area flicker. In the predominate United States display standard, the most visible problem is line flicker and visible line structure. This is due to the limited number of scan lines (i.e., 525 lines per frame). For the predominate display standard outside of the United States, the visible line structure is better due to a greater number of scan lines (625 lines). Large area flicker, however, is more obvious due to a refresh rate of only 50 Hertz. This is especially the case with large-screen televisions whose brightness and contrast is very high. With the demand for large-screen displays increasing, these problems will only become even more apparent and, hence, more critical to overcome.
The video signal provided to a video display system can be in the form of a composite video signal. A composite video signal can be an NTSC signal (the predominate United States standard), a PAL signal, or any other such signal as known to those in the art. NTSC stands for National Television Standards Committee and defines a composite video signal with a refresh rate of about 60 half frames (interlaced) per second. Each frame contains 525 lines and can contain 16 million different colors. A composite video signal provided as an input can also be a signal for a high definition ready television, that can provide much better resolution than current television standards based on the NTSC standard. PAL stands for Phase Alternating Line, the dominant television standard in Europe. Whereas, NTSC delivers 525 lines of resolution at 60 half frames per second, PAL delivers 620 lines at 50 half frames per second. The PAL and NTSC specifications are well-known to those in the art.
Various technologies have been developed to overcome these drawbacks of conventional television signals. For example, interlace-to-progressive conversion (also known as interlace-to-non-interlace conversion) can be used on a NTSC signal to remove line flicker and visible line structure. For a PAL signal, signal frame rate up-conversion is a means to increase the refresh rate to suppress large area flicker. Up-converting a 50 Hertz interlaced input to, for example, a 100 Hertz interlaced output is a widely accepted practice and can provide good picture quality on an interlaced PAL display system. However, the cost of a 100 Hertz PAL display can be very high. This is because a 100 Hertz PAL display requires a different, more expensive, picture tube and a more complex scan circuit than a typical 50 Hertz PAL television system. For example, today a 29-inch, 100 Hertz PAL television in a Chinese market costs approximately $1,000, while a conventional 29-inch, 50 Hertz PAL television typically costs around $250.
The higher cost of a 100 Hertz PAL television as described is due mainly to the complex scan circuit and picture tube. In a PAL television, large area flicker is a serious problem. To remove the flicker, the frame rate can be increased, e.g., to 100 Hertz as discussed above, which is widely accepted but expensive. It has been found, however, that large area flicker can be diminished even with smaller incremental refresh rate increases over the 50 Hertz refresh rate typical for a PAL system. A 60 Hertz or higher refresh rate has been found to reduce large area flicker such that for a live picture there is very little difference in quality between a 60 Hertz and 100 Hertz refresh rate PAL television output signal. A television with an up-converted interlaced 60 Hertz or higher vertical refresh rate output (less than 100 Hertz) from a 50 Hertz input PAL signal will have a scan rate that is much lower compared to that of a 100 Hertz vertical refresh rate television system. The cost for this kind of television system will thus be greatly reduced as compared to a 100 Hertz output system, while providing a comparable quality display signal. This is because the same tube and scan circuit can be used for a 60 Hertz or a 75 Hertz up-converted system as can be used with a 50 Hertz PAL television system.